Columbium treated, non-aging, vacuum degassed low carbon steel and method for producing same

ABSTRACT

A process of producing non-aging, low carbon steel having substantially no yield point elongation in the annealed condition and freedom from critical grain growth. A molten steel having an analysis typical of steel intended for rimmed or killed drawing steel is vacuum degassed to decarburize to a maximum carbon content of about 0.015%, and columbium (niobium) is added in an amount at least sufficient to combine with the carbon present in the steel. The cast material is hot rolled, finishing at 1500.degree. - 1700.degree.F (about 1,090.degree. 1,200.degree.K) and coiled at a temperature of about 1500.degree.F (about 1090.degree.K) or less. The columbium addition retards the rate of recrystallization of the cold rolled product, and a wide spectrum of mechanical properties can be obtained in the final product by control of the final annealing time and temperature within the range of 1000.degree. to 1700.degree.F (about 810.degree. to 1200.degree.K). A preferred product is cold rolled and annealed strip suitable for deep drawing, porcelain enameling, hot dip metallic coating and the like, containing at least about 0.025% uncombined columbium at the hot rolling stage, as determined by analysis at room temperature, which has an average plastic strain ratio of at least 1.8, and a uniform grain size between ASTM 8 and 10.

United States Patent (191 Elias et al.

Ill 3,876,390

[ 1 Apr. 8, 1975 l l COLUMBIUM TREATED. NON-AGING.

VACUUM DEGASSED LOW CARBON STEEL AND METHOD FOR PRODUCING SAME [75]inventors: James A. Elias, Middletown; Rollin E. Hook, Dayton. both ofOhio [73] Assignee: Armco Steel Corporation,

Middletown. Ohio [22] Filed: Aug. 6, i973 [2l] Appl. No: 386.l5l

Related U.S. Application Data [60] Division of Ser. No. H.077. Janv l8.l97l Pat. No. 3 76l.324, which is a continuation-in-part of Ser. No.l5.4l5. March 2. I970. abandoned Primary liramincrL. Dewayne RutledgeAssistant Eruminer-Arthur .I. Steiner Attorney. Agent, or Firm-Melville.Strasser. Foster & Hoffman [57] ABSTRACT A process of producingnonaging. low carbon steel having substantially no yield pointelongation in the annealed condition and freedom from critical graingrowth. A molten steel having an analysis typical of steel intended forrimmed or killed drawing steel is vacuum degassed to decarburize to amaximum carbon content of about 001571. and columbium (niobium) is addedin an amount at least sufficient to combine with the carbon present inthe steel. The cast material is hot rolled, finishing at lS00 |700F(about l.090 1.200K) and coiled at a temperature of about l500F (aboutl090K) or less. The columbium addition retards the rate ofrecrystallization of the cold rolled product, and a wide spectrum ofmechanical properties can he obtained in the final product by control ofthe final annealing time and temperature within the range of I000 tol700F (about 810 to l200K). A preferred product is cold rolled andannealed strip suitable for deep drawing. porcelain enameling. hot dipmetallic coating and the like. containing at least about 0.025%uncombined columbium at the hot rolling stage. as determined by analysisat room temperature. which has an average plastic strain ratio of atleast 1.8. and a uniform grain size between ASTM 8 and 10.

4 Claims, 10 Drawing Figures PATENTECAPR 8i975 3.876.390

I I I I 20 4O 60 60 I00 I20 TIME MIMJ TE 5 HA RDNES5 Rb Q RIM MED FIG. 2

Y I I I COLD REDUCTION RHENTEDAPR 8 i975 '24 z STRAIN Ti TREATED w w m 5m w m PATENTED &975

SHEET 5 BF 6 PATENTEBAPR'BxQYs seam s or 55 9 Q Q G 6 s 0 o Q Q G G O@90 m6 -llllllilGlIIIO. I l l l l i t ill 0 G 1 d 0 0 u 0 2 J lumaamwwWWLMmmLm WT: PCT Cb WCOMBINED l COLUMBTUM TREATED. NON-AGING.VACUUM DEGASSED LOW CARBON STEEL AND METHOD FOR PRODUCING SAMECROSS-REFERENCE TO RELATED APPLICATION This application is a division ofapplication Ser. No. 107.077. filed Jan. 18. l97l. now U.S. Pat. No.3.761.324. which in turn is a continuationin-part of Ser. No. l5.4l5.filed Mar. 2. 1970. now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to non-aging low car bon. columbium-treated steelha\ing no yield point elongation in the annealed condition. which hasexcellent surface characteristics and substantial freedom fromnon-metallic inclusions and a wide spectrum of mechanical properties.and to a method for Producing the steel. While the term columbium isused herein. it should be understood that niobium is the same element.Although not so limited. the steel of the present invention in the formof sheet stock has particular utility in deep drawing and stretchingoperations. in metallic coating processes. and in the production ofenameled steel.

2. Description of the Prior Art Both carbon and nitrogen give rise toyield point elongation in low carbon steels which have beenrecrystallization annealed. but strain aging which results in a returnof yield point elongation after temper rolling in such steels is usuallydue to nitrogen. Such strain aging is prevented by adding aluminum whicheliminates nitrogen from solution by formation of aluminum nitride. lfaluminum stabilized steels are subjected to high temperature aftertemper rolling. carbon will cause strain aging unless it also is removedfrom solid solution. Early workers in the art have stated that elementssuch as titanium. columbium, vanadium. zirconium. and chromium. if addedin sufficient amounts to combine with all the carbon present in thesteel. will eliminate aging and yield point elongation. Such elementshave a strong affinity for carbon and form stable carbides. therebyremoving soluble carbon from ferrite to such a low level that the asannealed yield point elongation is eliminated and strain aging iseliminated as well. The literature has indicated generally that theeffectiveness of such elements in preventing aging increases withincreasing affinity for carbon in the order chromium, Zirconium.vanadium. columbium and titanium. See Journal of Iron and SteelInstitute. 142. pages l99-22l ([940); Iron and Steel, June 1963. pages326334.

Thus. titanium has been considered the most effective element ineliminating aging and yield point elongation in low carbon steels, withcolumbium considered almost as effective, and other elements such asvanadium and chromium considered somewhat less effective.

U.S. Pat. No. 3.l83,078. issued May ll, l965. to T. Ohtake et al..discloses a process for producing nonaging enameling iron having gooddrawability. This process involves producing a molten steel containingless than 0.04% carbon and an analysis otherwise comparable toconventional rimmed steel (except for a preferred manganese content of0.05% maximum), vacuum degassing the molten steel to reduce the carboncontent to less than 0.02% less than 0.020% sulfur and 0.002 to 0.007%nitrogen, adding aluminum and titalll nium in amounts sufficient tocombine with the carbon. nitrogen and sulfur present in the steel. Inthe preferred practice some aluminum is added first in order to combinewith residual oxygen and nitrogen. thereby making most of the titaniumavailable for combination with carbon. sulfur and any residual nitrogennot combined with aluminum.

French Pat. No. l.5l [.529 granted Dec. 18. l967. to Yavvata Iron andSteel Co. Ltd. (the assignee of the above mentioned U.S. Pat.) disclosesa process similar to that of the U.S. Pat. for the production ofcoldrolled sheet stock having good deep drawing and stretching properties.In the process of this French patent a molten steel is subjected tovacuum degassing with the addition of aluminum as a deoxidizing agent toproduce a degassed steel containing less than 0.0209 carbon and lessthan 0.015% oxygen. Titanium is added in a weight ratio of4: l to thecarbon. and the degassed steel is then cast. hot rolled with a finishingtemperature above 780C (1.053K). cold rolled at a reduction rate above3071. and finally annealed at a temperature be tween 650 and l.000C (923and l273K). The resulting sheet stock is stated to have a strong {I l l}orientation normal to the sheet surface. or cube-on-corner texture. andto have a plastic strain ratio (r value) ranging from about 1.75 to 2.47depending on the processing used. The ASTM grain size ranges from 7.5 to10.

The r values set forth in the Yawata French patent are not identified asto which r value is designated. In any event. titanium-bearing steelsproduced by similar processing by applicants and others in the UnitedStates indicate that average r values above about 2.0 cannot beobtained.

in the present application the average plastic strain ratio Fis thestandard calculated as F fiilrflongitudiriall r( transverse) 2r(diagonal )1.

While the addition of titanium to a vacuum degassed steel results in aproduct having non-aging properties and no yield point. the productnevertheless suffers from a number of disadvantages. Since titanium is astrong nitride. oxide and sulfide former. as well as a carbide former. alarger addition of titanium than the amount theoretically necessary tocombine with carbon is required because of the reaction of part of thetitanium with nitrogen. oxygen and sulfur present in the steel. Thus,although the theoretical stoichiometric ratio of titanium to carbon isabout 4:1. this must be increased initially to a ratio of about 8:lbecause titanium reacts with the residual sulfur and nitrogen in thesteel. In addition, still more of the titanium is lost as a result oftitanium oxide formation which goes into the slag. It has therefore beenfound that in commercial practice titanium must be added in a weightratio to carbon of as high as 16:] in order to obtain a non-aging steelhaving no yield point. The titanium recovery may thus be on the order of50 to under such circumstances.

The formation of oxides. nitrides and sulfides of titanium in the steelresults in objectionable non-metallic inclusions of these compounds andadversely affects the surface quality of the product.

Titanium in solution in the steel may prevent the healing of hot cracks,as is known to be the case with aluminum.

The great affinity of titanium for oxygen in the air also renders themolten steel less fluid during casting.

Moreover. the titanium bearing steels ofthe type dis' closed in theabove mentioned French patent have in herently low strength. notexceeding about 20.000 psi yield strength (138 MN/m which cannot beincreased substantially by the final annealing treatment.

Due to the above disadvantages and to the increased cost resulting fromthe practical necessity of adding up to four times the theoreticalamount of titanium needed. vacuum degassed. titanium-treated steels havenot gained commercial acceptance over rimmed and killed steels for deepdrawing. stetching. coating. or enameling applications.

It has previously been reported by Abrahamson et al. in TransactionsMetallurgical Society of AIME." Vol. 2l8. Dec. 1960. pages H] 1104. thatcolumbium and zirconium substantially retard the rate ofrecrystallization during annealing of cold rolled material in comparisonto alloying elements such as titanium and chromium. These findings werebased on one-hour anneals with increasing temperature throughout each anneal. However. no practical benefit or advantage has ever previouslybeen derived from this knowledge.

SUMMARY The present invention provides a non-aging low carbon steelhaving substantially no yield point elongation and freedom from criticalgrain growth in both the hot rolled and the cold rolled and annealedcondition. which avoids the disadvantages of the prior arttitanium-bearing steels and moreover exhibits a high degree of nearcube-on-corner crystalline orientation. and superior Furlues. and arelatively small grain size which is stable over a broad temperaturerange. Furthermore the material is producible with a broad spectrum ofproperties in either the hot rolled or cold rolled conditions. Themethod ofthis invention comprises the steps of providing a molten steelhaving a maximum carbon content of about 0.05% and sufficient manganeseto combine substantially completely with the sulfur pres ent in thesteel; vacuum degassing the steel to a carbon content of about 0.0l57rmaximum. an oxygen content of about 0.0[071 maximum. and a nitrogencontent of about 0.0129? maximum; adding columbium in an amount at leastsufficient to retard the recrystallization rate of the steel whensubsequently solidified; casting and solidifying the degassed steel; hotrolling the steel to band thickness, finishing at a temperature of aboutl.500 to l.700F (about l,090 to 1.200K); and coiling at a temperature ofabout l,500F (about 1.090K) or less. The hot rolled product is highlydesirable for some applications as coiled or as annealed. Usually thehot rolled product will be pickled and cold reduced to final gauge.followed by a final anneal at a temperature and for a length of timeselected to produce a desired strength level and ductility in thefinished strip or sheet.

The hot rolled product may be used as coiled or may be subjected to afinal anneal within the temperature range of 1.350 to l.700F (aboutl.005 t0 1,200K). The cold rolled product will ordinarily be subjectedto a final anneal within the temperature range of l.0OO to l.600F (about810 to l.l45K). In either case the final anneal may be either batch orcontinuous. or as incidental but necessary to hot dip metallic coating.and may range from seconds to about 16 hours. For maximum hardness andstrength in the hot rolled product the coiling temperature should rangebetween about 940F and about 1.300F (about 775 and carbon 0.002 to0.015% colum- 0.02 to 0.30% bium" manganese 0.05 to 0.60% sulfur up to0.035% oxygen up to 0.0100; nitrogen up to 0.012% aluminum up to 0.08%phosphorus residual silicon residual remainder substantially iron. *1antalurn is commonly present as an impurity in columbium and in smallamounts is not undesirable. and \\ill act similarly.

The present invention constitutes a discovery that columbium isunexpectedly superior to titanium both from the processing and productstandpoints in a number of significant respects.

For example. applicants have discovered that the previously reportedslow recrystallization rate of the columbium-bearing cold rolled steelof this invention permits the attainment ofa broad spectrum ofmechanical properties if certain processing controls are ob served.Recrystallization ofthe cold rolled structure of the steel of thisinvention is unlike any other low carbon steel. Recrystallization beginsat the strip surfaces and proceeds inwardly such that a banded structureis frequently seen in a partially recrystallized product. Alternatively.the time and temperature of the final anneal may be so selected as toresult in substantial recrystallization throughout the strip.

In the process of the present invention. sulfur is com bined withmanganese. and for this purpose the manga nese content preferably ismaintained at a weight ratio to sulfur ofabout 7: l Aluminum may beadded to combine with oxygen and nitrogen, and when so added the weightratio ofaluminum to oxygen is preferably 1. [2:] while the ratio ofaluminum to nitrogen is preferably 2:1. Since enough aluminumandmanganese are present to combine effectively with sulfur, oxygen andnitrogen, and since columbium has less affinity for oxygen. sulfur, andnitrogen than does aluminum at the temperatures involved. substantiallyall the columbium added during or after the degassing step and after thealumi num addition is available to combine with carbon. Much higherefficiency results. and columbium recoveries of to obtainable.

Aluminum may be omitted. or another nitride former such as titanium maybe substituted. If a nitride former is omitted, the nitrogen will becomined with columbium. If tight coil annealng in a nitrogen-hydrogenatmosphere is to be practiced. aluminum should be added since the steelpicks up nitrogen from the annealing atmosphere which would combine withcolumbium if insufficient uncombined aluminum is present therebyresulting in a product having an as annealed yield point elongation ifnitrification occurs to the degree that uncombined nitrogen is present.When open coil annealing is to be practiced. this precaution need not beobserved.

The use of columbium in place of titanium. the addition of sufficientaluminum to combine with oxygen and nitrogen and the maintenance ofsufficient manga nese to combine with the sulfur present in the steelresult in a material having surface characteristics superior to that oftitanium-bearing steel. and the nonmetallic inclusions are substantiallyeliminated in the process of the present invention by removal in theslag. It is well known in the art that titaniumbearing steels contain anobjectionable amount of inclusions and have poor surface quality.

The steel of the present invention has consistently higher plastc strainratios than do titaniunrbearing steels similarly processed.

It has been found that high plastic strain ratios are obtained whencolumbium is added in an amount greater than that required to combinewith carbon and any uncombined nitrogen; i.e.. when columbium is presentin the hot rolled thin bar in uncombined form (apparently in solidsolution) a texture is obtained which. after subsequent cold reduction.recrystallizes upon annealing into a final product having a high degreeof near cube-on-corner orientation such as 554} and {322}. Morespecifically. average plastic strain ratios of 1.8 or higher areobtained when at least 0.025% by weight of columbium is present inuncombined form in the hot rolled thin bar, as determined by actualsheet analysis at room temperature.

The steel of the present invention. whether cast in ingot form orcontinuously cast. can be hot rolled by standard practices and onconventional rolling equipment. thereby assuring low processing costsand avoidance of capital outlay for new plant equipment.

The atomic weight of columbium is 92.9l and hence the theoreticalstoichiometric ratio for complete reaction with the carbon (atomicweight 12.01) present in the steel is about 7.75: l Titanium has anatomic weight of 47.90 and the theoretical stoichiometric ratio oftitanium to carbon is thus about 4: 1. It has been found that acolumbium to carbon ratio of l0:l. or preferably I211. will produce amaterial which is completely non aging and which has no yield pointelongation. A columbium to carbon ratio of 8:1 may produce a materialwhich has marginal stability in that it might show some yield pointelongation under certain annealing conditions. However. a steel whichdoes have some yield point elongation can be subjected to a standardtemper rolling step which will eliminate the yield point. and thematerial will be non-aging because of the low carbon content.Alternatively such a material could be decarburized after cold rolling.either in a separate step or as an incident to the finalrecrystallization anneal. to produce complete stability. A steel havinga columbium to carbon ratio of less than 8:] is thus considered withinthe scope of this invention. In contrast to this. when it is realizedthat a ratio of titanium to carbon of as high as 16:1 is required inactual practice. because of its reactivity with other elements and thelow recovery, despite a theoretical stoichiometric ratio of 4:1, themarked superiority in effectiveness and efficiency of columbium overtitanium is apparent.

Although the high cost of columbium would appear at first blush topreclude its use in a low carbon steel for applications such as coating.enameling and the like. applicants have found that the use of columbiumresults in reduction of processing costs. elimination of someoperations. lower rejections and higher yields which more than offsetthe cost of the columbium addition and the vacuum degassing step.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is hereby made to theaccompanying drawings wherein:

FIG. 1 is a graphic representation of recrystallization response as afunction of annealing time and hardness of columbium-bearing steels incomparison with titanium-bearing steels:

FIG. 2 is a graph showing the relationship between T and percent coldreduction for rimmed. aluminum killed. titanium and columbium treatedsteels:

FIG. 3 is a graph showing the effect of varying columbium to carbonratios on yield point elongation and yield stress;

FIG. 4 is a graphic comparison of yield strengths of columbium bearingsteel of the invention with titanium-bearing steel and a commercialgrade enameling steel after straining and firing.

FIGS. 5-9 are photomicrographs at [00x magnification of sections of asteel of the invention showing the mechanism of recrystallization duringfinal annealing; and

FIG. 10 is a graph showing the relationship between Fund the amount ofuncombined columbium present in the hot rolled product.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A heat of steel may be meltedin an open hearth. basic oxygen furnace. or electric furnace. having atypcal but non-limiting analysis of steel intended for rimmed or killeddrawing steel (0.02 to 0.05% carbon. 0.l to 0.35% manganese. 0.01 to0.020% sulfur. 0.00l to 0.010% nitrogen. and balance substantiallyiron). The molten steel is subjected to decarburization by vacuumdegassing in conventional equipment. prefera bly with argon bubbling toassist in removal of impurities and to avoid temperature stratification.some aluminum is preferably added before degassing in order to stun theheat. i.e.. to prevent excessive evolution of gases. Other deoxidants.such as silicon. may also be added in small amounts.

The balance of the aluminum is added preferably during the vacuumdegassing but after decarburizing.

The addition of aluminum above the amount necessary to combine withnitrogen and oxygen may not be desirable since it may adversely affectthe quality of the final product. More specifically, the presence ofexcess aluminum in the product may interfere with the healing ofhot-short cracks which may is present, although hotshortness is avoidedby ensuring a manganese content high enough to combine substantiallycompletely with the sulfur present in the steel. For this purpose aratio of manganese to sulfur of about 7:1 should be observed. but highermanganese contents can be be tolerated and would not adversely affectthe final properties.

Columbium is added after the aluminum preferably during degassing, or inthe ladle or mold if proper distribution means are provided.

A columbium to carbon ratio of l2:l is preferred in order to ensurecomplete and permanent removal of carbon by formation of columbiumcarbide. However,

still higher columbium ratios may be utilized. in order to promote grainorientation and desired mechanical properties in the final product.

Silicon is preferably not added. but minor amounts can be tolerated.Other elements in normal residual amounts can also be tolerated.

The degassed steel should have the following preferred analysis. and thecomposition of the final product will also be substantially the same:

Latrhnn (H105 to 1|.1lllli columbium 0.118 to 0.12; manganese (l. 10 to0.55) sull'ur up to 0.02; o\ \gen up to 0.004; nitrogen up to 0.0110;aluminum 01115 to 0.020% phosphorus up to (1.010; silicon up to t1 t1l5;

remainder substantially iron. except for incidental impurities.

The degassed and treated steel may then be cast into ingot molds. or maybe strand cast by conventional practices.

Where continuous hot rolling is to be practiced. the ingots are reducedto slab thickness. reheated if neces sary. hot rolled to band thickness.and coiled.

A conventional hot band finishing temperature of 1.500 to 1.700F (about1.090" to 1.200K) is preferred and is not critical in the practice ofthe present invention. However. a Finishing temperature below about1.500F (about 1.090K1 results in higher power requirements. and it ismore difficult to obtain the desired thickness. A finishing temperaturesubstantially abote about 1.700F (about 1.200K) requires higher rollingspeeds. and a thicker and hotter bar is sent into the finishing stands.

A rapid quench to a coiling temperature between about l.l00 and 1.300F(about 865 and 975K) is preferred although higher or lower coilingtemperature extending to the practical limits may be practiced. lngeneral. coiling at higher temperature tie. up to 1.500F or about1.090K) results in a softer product. while coiling at lower temperatures(i.e.. down to 940F or about 775"K) results in a harder product.Quenching to such low coiling temperatures is difficult to achieve onexisting equipment.

As an adjunct or alternative to coiling at a relatively hightemperature. a continuous or batch anneal of the hot rolled band can becarried out at a temperature up 8 to about 1.750F (about 1.230K) inorder to obtain a hot rolled product having the maximum degree ofsoftness and ductility.

The coiled material is then pickled and cold rolled substantially tofinal gauge. preferably without intermediate annealing. in accordancewith conventional practice. The cold reduction may be on the order of toand does not constitute a limitation on the process ofthe invention.Higher degrees of cold reduction up to result in higher Tvalues.

The cold rolled strip is then subjected to a final anneal in aprotective atmosphere. which may be either continuous or batch.

It will be understood that the hot rolled band or thin bar is a productwhich is sold commercially. and its properties are dependent on thecomposition of the steel and the coiling temperature, i.e., the rate ofcooling from the finishing temperature to the coiling temperature andthe degree of annealing which occurs in the compact coil as it coolsslowly. Unlike conventional low carbon or titanium-treated steels. thehot rolled product can be produced with a wide spectrum of mechanicalproperties ranging from high strength and hardness to moderate and lowstrength and accompanying high ductility. Of course the plastic strainratio will be substantially 1.0. as for any hot rolled. low carbonsteel.

Table 1A below illustrates the range of mechanical properties of 0.100inch (2.54 mm) thick hot rolled thin bar produced in an experimentalmill processed ton 145 metric ton) open hearth melted and vacuumdegassed heat containing 0.11% columbium and 0.005% carbon(columbiumzcarbon ratio of 22:1 Table 18 below illustrates the range ofmechanical properties of 0.077 inch (1.96 mm) thick hot rolled thin barproduced in an experimental mill processed ton 154 metric ton) electricfurnance melted and vacuum degassed heat containing 0.14% columbium and0.008% carbon (Cb:C ratio of 17:1). Quenching from the hot rollingfinishing temperature of about A 1.600F (about 1.145K) to a low coilingtemperature of 1.100F (about 865K or below results in a fine dispersionof columbium carbide precipitates which contribute to the high strengthand hardness developed in the hot rolled product. while the employmentof higher coiling temperatures. from l300 to 1,500F (about 975 to1.090K) results in a coarser dispersion of these precipitates and lowerstrength and hardness.

TABLE IA Hot Rolled Thin bar (0100" or 2.54 mm thick) Mill Produced andProcessed Steel Containing 0.1 1% Columbium and 0.005% Carbon TensileYield Coiling Hardness Strengjh Strength Z Elong. Temp. R ksi N/m ksiMN/m in 2" BF 6K TABLE 113 Hot Rolled Thin Bar (0.077" or 1.96 mm thick)Mill Produced and Processed Steel Containing 0.14% Columbium and 0.008%Carbon Tensile Yield Coiling Hardness Strength Strength Z Elong Temp. Rksi MN/m ksi MN/m In 2" OF OK 940 775 76 67.8 468 48.7 336 25 1100 86575 65.0 449 46.2 319 30 1300 975 60 52.6 364 31.1 214 40 Regardless ofthe strength and hardness produced by quenching from the finishingtemperature to a low coiling temperature. the hot rolled band can berendered soft and ductile by post annealing. lf the band is annealed inthe ferritic range (below the A. temperature of about 1.670F or 1.183K).no grain growth occurs but the columbium carbide precipitates arecoarsened. and a softer and more ductile product is produced. Annealingsomewhat above the austenitization temperature results in a coarsergrained transformed ferrite and an even softer product than can beobtained by annealing at a temperature in the ferritic range. Table Abelow illustrates the effect of such post annealing temperatures on hotrolled material which had been coiled at 1100F (about 865K).

TABLE 11A is not subject to coil breaking during winding onto orunwinding from a mandrel. Hence the hot rolled band can be hot dipmetallic coated on continuous coating lines without coil breaks; thishas been practically im- 5 possible with steels of the prior art. Thecoated strip can be roller or stretcher-leveled to produce a high degree of flatness without undergoing fluting or stretcher strains. Thesteel does not exhibit stretcher strains during forming. which can causebreakage and/or poor surface appearance in conventional low carbonsteels. In the cold rolled and annealed strip a wide spectrum ofproperties can be produced ranging from high strength with limitedductility to moderate strength with high ductility and high Fvalues.which are required 15 for good deep dravvability. The properties of thestrip Post Annealed Hot Rolled Thin Bar (0.100" or 2.54 mm thick) MillProduced and Processed Steel Containing 0.l l r Columbium and 0.005%Carbon Tensile Yield Post Annual Grain Size Hardness Stren th Stren th95 Elongv Condition ASTM R, ksi E/lN/m" ksi N/m in 2' Continuous StripAnneal in Ferritic Range (1600F-l I45Ki 8-9 46 46.0 3 l '7 25.0 172 47Continuous Strip Anneal above Austenitization Temperature (l700F-l200K)5-6 40 4 l ,0 283 24.0 loo 4) The sluggish response in softening of thesteels of the invention makes it possible to retain the hot rolledproperties after hot dip metallic coating, even where the hot rolledband is subjected to relatively high temperatures, such as 1,350F (aboutl,005K), for a short time, as in aluminum coating. This is illustratedin Table "8 below, where material having a columbium to carbon ratio of17:1 was coiled at 940F (about 780K). (The properties before coating aregiven in Table IB above.)

TABLE IIB Aluminum-Coated Hot Rolled Thin Bar (0.077" or [.96 mm thick)Mill Produced and Processed Steel Containing 0.14% Columhium and 0.008%Carbon Tensile Yield Hardness Stren th Stren th 9 Elong. Condition R ksiN/m" ksi fiIN/m in 2" As Coated (l350F or l005K Strip Temperature)Stretch and Roller Leveled 74 65.0 449 54.0 373 20 The hot rolled bandor thin bar of the present invention does not exhibit yield pointelongation and hence greater than in any low carbon ferritic steel,either rimmed. aluminum-killed or titanium-treated. The

graph of FIG. 1 illustrates the recrystallization response. as afunction of decrease in hardness. with time at annealing temperatures ofl.200F and 1300F (about 920 and 975K) for columbium treated andtitanium-treated steels.

Moreover. the formation of columbium carbide precipitates providesinherent strengthening of the steel which can also be controlled byproper selection of the final annealing conditions. Table IIIillustrates the spectrum of tensile and yield strengths which aredeveloped by annealing at l.200f (about 920K) and 1.300F (about 975K).respectively. a mill produced I60 ton (l45 metric ton) open hearth heatcontaining 0.l W: columbium and 0.005% carbon. vacuum degassed, pouredinto ingot molds. hot rolled to 0.100 (2.54 mm) thickness. coiled at1,300F (about 975K) and cold reduced 65% is compared to atitanium-treated steel and to conventional aluminum-killed and rimmedsteels. The superiority in Tvalues ofthe steel of the invention. withinthe cold reduction range of 50 to 90% is apparent.

The previously discussed sluggish softening response in steels of thepresent invention provides potential for the production of full hardmetallic coated strip which has heretofore been impossible to producewith aluminum coatings. A full hard product is one having cold reducedproperties such as a yield strength of 90 ksi (62] MNlm or higher ascoated. During metallic coating the strip is usually heated to 1.250F(about 950K) or higher to clean the surface and to bring it to thecoating temperature. Prior art rimmed, killed or titaniumtreated steelsrecrystallize very rapidly at temperatures near l,200F (about 920K) andthus lose full hard properties. The alloy of this invention can be an-TABLE III Spectrum of Properties Developed on Annealing l200F (920K)l300F (975K) Annealing Time T.S. 0.5% Y.S. Z Elong. T.S. 0.5% VS. 9%Elong.

Hrs. ksi MN/m ksi MN/m in 2" ksi MN/m ksi MN/m in 2" l 7L3 492 67.2 464[0.5 49.2 340 27.7 l9l 39.7

4 58. 402 48.0 332 Ill 46.] 3H? 2H) I45 45.6

In 53.6 370 40.0 276 29.2 45.4 313 20.2 I39 48.2

Yield Point Elongation 07:. all conditions.

The properties developed by annealing following cold reduction arerelated to and dependent on the strength and hardness of the hot rolledband or thin bar. The greater the hardness exhibited by the hot rolledthin bar before cold reduction. the greater will be the strengthexhibited by the annealed strip for any given annealing condition. Hotrolled thin bar processed to exhibit less than maximum hardness. e.g.,by coiling at a relatively high temperature (e.g.. l,300F about 975K orabove) or by post annealing. will have more moderate strength andgreater ductility after cold reduction and annealing. The effect of thinbar hardness on mechanical properties after cold reduction and annealingis shown in Table IV. for an experimental mill-produced heat having acolumbium:carbon ratio of 22zl.

to simulate commercial controlled grain practice, with a finishingtemperature of 1.600F (about l,l45K) and a coiling temperature of l,l0OF(about 865K).l The hot rolled band was cold reduced 60% and an-l TABLEIV.EFFECT OF HOT ROLLED THIN BAR HARDNESS ON COLD ROLLED AND ANNEALEDPROPERTIES 1,200 F. anneal (920 K.)

1.300 F. anneal (975 K.)

Percent elong. Percent elong. T.S. (k.s.l.) 0.5% Y.S. (k.s.t.) in in. TS. (k.s.1 0.5% Y.S. (k.s.l.) in 21m.

A B C A B C A B C A B C A B C A B C Hardness, RB

NOTE.T0 obtain MN/rn', multiply by 6.9.

The effect of cold reduction on the plastic strain ratio is graphicallyillustrated in FIG. 2 where a steel of this invention having a columbiumto carbon ratio of I71] Coiled 1,100" F. (about 810 FL), cold reduced65%, annealed. Coiled 1.300 F. (about 975 K.). cold reduced 05%.annealed. Coiled 1,301? F. (about 975 K.). annealed 1,600 F. (about1.145 K.). cold reduced 65% annealed.

from FIG. 3 that in steels of the specified carbon con-j tent which havebeen subjected to the process of the present invention. a columbium tocarbon ratio of 8:1 or greater renders the steels of yield pointelongation even when sulfur. oxygen and nitrogen are present. Since thestoichiometric ratio of columbium to carbon in columbium carbide is7.75: l. the graph of FIG. 3 illustrates the high efficiency andeffectiveness of columbium in combining selectively with carbon andremoving carbon from solution.

Annealing conditions also affect yield point elongation oflaboratory-produced steels within the range of columbium to carbonratios of about 7:1 to 10:1. Thus, in a laboratory-produced steel havinga columbium:- carbon ratio of about 7:1. annealing at 1.300F (about975K) produced transient instability for an annealing time up to about 8hours. but continuation of the an' neal up to 16 hours resulted inreducing the yield point elongation back to a value ofless than 1%. Onthe other hand. annealing at temperatures in the range of l,400 to1.600F (about 1.035 to 1145K) resulted in both transient and persistenttypes of instability for annealing times up to 16 hours.

In a laboratory-producd steel having a columbium to carbon ratio of 10:l. annealing within the temperature range of 1,400" to 1.500F (aboutl.035 to 1.090K) produced temporary instability for an annealing time ofabout 2 hours. but when continued for a time up to 8 hours. the yieldpoint elongation was reduced to a value of on the other hand. annealingat 1.600F (about l.l45l() produced both transient and persistentinstability for annealing times up to 9 hours.

In contrast to this, in a laboratory-produced steel having a columbiumto carbon ratio of about 12.5:1, the material was completely andpermanently free of yield point elongation under annealing conditionsranging from temperatures of 1,300F to 1.600F (about 975 to 1.145K) fortimes of minutes to 16 hours.

Transient instability may only be a phenomenon 14 1,145K However. asindicated previously. the material can be temper rolled to eliminate theyield point elongation. and the product would thereafter be nonaging.

One of the most significant properties of the steel of the presentinvention is freedom from critical grain growth. which makes thematerial particularly useful for enameling steel. The firing ofporcelain enamel coated drawn parts results in critical grain growthwhen conventional or titanium-treated steels are used. and this has beena problem of long standing. Critical grain growth results in an extremeloss in strength because of the large ferrite grain size which developsalong the critically strained regions of a drawn part in the annealingwhich occurs as a result of firing the applied frit. Applicants havefound that the columbium-treated steels of the present invention notonly show freedom from critical grain growth but even show enhancedstrength as a result of critical straining of the drawn parts. Table Vand FIG. 4 compare an experimental columibium-bearing mill produced heatof the steel of the present invention with a titanium-bearing enamelingsteel of the composition disclosed in the above mentioned U.S. Pat. No.3.133078, and a standard commercially available grade of enameling steelsold under the registered trademark UNIVIT. The columnium-treated steelis the same heat as that described in Table 111 above. The graph of FIG.4 shows that the steel of the present invention gradually increases instrength with inceasing degrees of strain up to 16% and never decreasesto the orginal strength, while the titani' um-treated steel increases instrength when strained up through 8% but exhibits a loss in strengthbelow the original strength when strained 12% or more. The commercialenameling steel exhibits a loss in a strength as a result ofeven theslightest degree of strain. Moreover. Table V shows that the grain sizeof the steel of the present invention remains constant even whenstrained TABLE V Critical Grain Growth After Firing at 1450F (about1060Kl for 5 Minutes C b-Treated Armco UNlVlT Grade Mill Produced SteelTi-Treated Steel Enameling Steel 1 Strain ASTM ASTM ASTM Before Y.S. '1'Grain .S. "1 Grain (5. Z Grain Firing ksI MN/m YPE Size ksi MN/m YPESize ksi MN/m YPE Size 0 19.4 134 0 8 17.0 117 0 8-9 34.6 238 8.0 8-9 424.7 170 0 8 22.6 156 0 8-9 32.6 225 4.2 89 8 29.2 202 0 8 27.9 192 O 832.3 223 2.5 8-9 12 33.8 233 0 8 14.2 98 0 l 2 13.6 94 0 l 16 35.6 246 08 15.5 107 0 1-3 14.0 97 0 2- 3 20 29.2 202 O 8 15.0 103 0 3-4 13.7 95 03-4 24 25.6 l77 0 8 16.5 114 0.8 4-5 found in laboratory producedmaterials, probably as a result of the relatively rapid cooling ofingots and hot bands which results in very fine carbide precipiates.Such a phenomenon has not been found in a mill produced heat with amarginal columbium to carbon ratio.

The presence of a yield elongation in steels having a columbium tocarbon ratio in the range of 7:1 to 10:1 at annealing temperatures of1.500 to 1.600? (about l,090 to 1,145K) would minimize the value of thisin vention for use of such material in hot dip continuous coating withaluminum or zinc. since such a coating process involves annealing for ashort time at temperatures between 1,350 and l,600F (about l,005andbeyond 16 percent.

A preferred columbium treated steel of the present invention, containing0.11% columbium and 0.005% carbon was processed through the hot rollingand coiling stages and then subjected to a variety of subsequentoperations. The mechanical properties are set forth in Table VI below.It is significant to note that comparable strengths and elongations andhigh ?values can be obtained on cold reduced sheet by both batchannealing and hot dip metallic coating. The coated hot rolled productcan be produced with the same strengths and high elongation values asare obtained with cold rolled. batch annealed and/or coated products.

TABLE VI Mill Produced Drawing Quality Steel Containing 0.1 19'Columbium and (1.0056? Carbon 6% Yield Tensile Hardness StrengthStrength Elong.

Condition R, ksi MN/m" ksi MN/m" in 2" r Open coil annealed at 13X0F(about l020Kl 8 hrs. after 651' cold reduction to 20 gauge. then .1 5temper rolled 4i -14 2l.0- 145- 45.0 310- 45 48 1.95- l'nr flatness 22.0152 46.0 31-4 210 Box annealed at l37SF (about ll|ZllKl 11 hrs. afteras; cold reduction to 20 gauge. then 2''; temper rolled 1'4 200-- 138-45.0 310 44 2.1 for flatness 21.0 145 Zinc coated after 70; coldreduction to 22 gauge: slllp temp. 1500 lhll0F luhout 108 0 40 22.ll-152- 400- 314- 40-41 1.78 I145Kl 23.0 159 47.0 324 Zinc coated alter.Ill-t" (26,-; mml hot rolled band; strip temp.

l500 lbtlll F (about lll9l| 43 -47 22.() 152- 440- 304- 45 47 1.0ll-15l\') 25.0 172 45.0 310 Yield Point Elongation 0%. all conditions.

The correlation between average plastic strain ratio and the amount ofuncombined columbium in the hot rolled thin bar is graphicallyillustrated in FIG. 10. Data were obtained from a number ofcontinuously-cast heats and from a number of ingot heats, each typebeing subjected to the same processing conditions. The ingots or slabswere hot rolled with a finishing temperature of l.650F (l.l70K) andcoiled at l,200F (920K). The hot rolled thin bar ranged between 0.090and 0.100 inch (2.29 and 2.54 mm) in thickness.

The columbium carbon and aluminum contents were intentionally varied inthese heats, while the remaining elements were maintained constantwithin commercially practicable limits. More specifically, the totalcolumbium contents were varied between about 0.068%

and 0.25% carbon between 0.00227: 7r and 0.020%.

and aluminum between less than 0.002% and 0.070% Other elements werewithin the following ranges:

manganese 0.3 0.5% sulfur 0008 0.019% nsygcn 0.001 (LUV/r nitrogen 0.0040.003)? phosphorus and silicon residual rernn inder substantiall} ironThe amount of uncombined columbium was calculated by either of thefollowing two formulae, depending upon whether or not aluminum was addedto combine with nitrogen.

2. "/2 Cb,,,,,.,,,,,,, 7r Ch 7.75% C total where tulul m'id sol/ S 0 1ftitanium is used as a nitride former rather than aluminum. theseformulae can be appropriately modified to account for this substitution.

in FIG. 10 Fvalues are for the final product after 62% cold reductionand annealing at 1375F (1,020K),

while the percentages of uncombined columbium are calculated by formulae1 and/or 2 using percentage values of total columbium, total carbon,total nitrogen and acid-soluble aluminum for the hot rolled thin bar, asdetermined by sheet analysis at room temperature. It will of course beunderstood that the actual percent of uncombined columbium, or columbiumin solid solution, at the hot rolling temperature will not be the sameas that analyzed at room temperature. However, it has been found thatthere is a well defined relationship between Tand the uncombined Cbdetermined at room temperature.

As will be apparent from FIG. 10, a marked difference in rvalues occursbetween about 0.022% and about 0.026% uncombined columbium, and thecritical value thus appears to be about 0.025% uncombined columbium,above which Fvalues in excess of 1.8 can be obtained. One heat, having0.027% uncombined columbium, exhibited an r value of only L65, and thisexception to all the other data is not at present explainable.

Variations in total carbon, aluminum and nitrogen contents were found tohave relatively little effect on 'Fvalues, provided sufficient columbiumis added to provide an excess of at least about 0.025% uncombinedcolumbium, as determined in the hot rolled product, as

l7 18 will be apparent from a consideration of Table Vll beand l.380F(about l.020K) for 8 hours respectively Mechanical properties of thesesamples are set forth in TABLE V" Table Vlll below TABLE Vlll 91A1 1031N+113G0 5 Tom .S i Yield Tensile U y Strength- Stren th- 9% Elong.101mm (h I b Figure ksi MN/mksi /ll i/m in 2" r by formula 2 5 54.0 37267.0 462 I8 I00 .071 2.10 .005 .0031 .0001 10 0 43.0 1% 50.0 407 1.17.070 L97 .1198 .1103s .0048 7 31.0 214 53,0 360 33 54 .057 2.13 .079.0020 .0055 s 25.0 17: 414.0 331 40 1.94 .100 2.06 .15 .0053 .0053 031.0 145 400 31s 44 1.03 .01 7 3.1 1 .1: .0043 .0057 061 3.07 .1013.0030 000x .030 11w .001 .0000 .0003 A .(12; L07 .24 .0023 .0050 I5While the preferred practice of the process of the 3' {.12 presentinvention contemplates the step of quenching .010 1.04 .13 .0001 .0051the hot rolled material from a finishing temperature in Q3; t? mi; {353the range of 1.500 m 1.700F (1.090 m 1.2001 1 to 2, 1 1]} m1; 1 atemperature therebelow at a rate rapid enough to W5 vlltlfl W64 causeprecipitation ofcarbides in finely dispersed form. .027 1.115 .0014.0033 .0069

U I) H 020 it will be understood that the scope of the invention is '7;Al 1.93% N 1.12%; 0 not so limited and covers a product not produced in(gy 1 1 n 0% x m g this manner which nevertheless is fully stable byreason m 5 1,6" of the columbium addition. and which has great and .0343.13 .0 40 .0040 .0047 particular utility for drawing and/or stretchingapplica- .002 1.59 .073 .0047 .0062 m4 1 66 (m Um mmt1ons.enameltng.metallic coating and other uses 0 here ms 3 1,037 0 1good ductility. absence of critical grain growth. aging .102 1.03 .20.0022 .0042 z MI I) M: mm 059 1nd yield point elongatton lre required.036 1 g! U44 The embodiments of the invention 1n which an exclu- .0221.47 .094 .0076 .0075 30 sive property or privilege is claimed aredefined as fol- 0 1.44 .10 .010 00x4 lows. 0 1.48 .11 .011 .0053

g 0 1. Hot dip metalhc coated. hot rolled. low carbon The data of Tablclate to the Same heats pkmed I steel having substantially no yield pointelongation and The effect of addition f ffi i l bi to having tensilestrengths ranging from about to 70 ksi vide at least about 0.025%uncombined columbium in 35 (275 to 435 MN/m2l after Stretching androller the hot rolled product is confirmed by X-ray diffraction g themetallic Coating being Chosen fmm the C1355 studies. These show that thetextures of hot rolled. and Consisting 0f aluminum and Zinc. id lEBl wsifitiltg cold redu ed and an e l d d t 0n[uining n essentially of fromabout 0.002% to about 0.015% carleast about 0.025% uncombined columbiumare distin 4 from ilbOVe about (102571 to ub ut 0.30% Columguishablefrom the textures of comparable products 0 biumfrom about 0057' to aboutmanganese containing less than about 0.025% uncombined colum- Sulfur p 0bout 0.035%. oxygen up to about 0.0l0%. bium. nitrogen up to about0.012%. aluminum up to about n FIGS. 57 the banded structure frequentlyassoci- 0.080%. phosphorus and silicon in residual amounts. all atedwith incomplete recrystallization of the steels of 45 percentages beingby weight, and remainder u anthe invention is illustrated. These areetched sections. Ilkllly Wlth at east 0.025% by eight of uncomat l00Xmagnification. of a mill-produced and pro bined columbium being presentas determined by analcessed steel containing 0.l l% columbium and 0.005%ysis at room temperature and calculated from either of carbon. hotrolled to 0.100 inch (2.54 mm) thickness. ormu e 1 and 2 herein. coiledat l.330F (about 975K) and cold r duced 65% 2. Hot dip metallic coatedsteel as claimed in claim The figures show the gradual recrystallizationinwardly I, said steel consisting essentially offrom about 0.005% fromthe surfaces at 4. 8. and 16 hour stages of an anto about 0.0l0% carbon,from about 0.08% to about neal at l.200F (about 920K). This very unusualre- 0.l2% columbium. the weight ratio of columbium to crystallizationresponse is not explained although it is carbon being at least about10:1. from about 0. l0% to believed to be caused by the reduced freeenergy of 5g about 0.35% manganese. up to about 0.02% sulfur. oxsurfacematerial. This structure is not only a distinygen up to about0.004%.nitrogen up to about0.006%. guishing characteristic ofthe steelof this invention. but aluminum from about 0.0l5% to about 0.020%. subitalso has advantageous aspects. For example. a parstantially all thealuminum being combined with oxytially recrystallized product has highstrength and formgen and nitrogen. phosphorus up to about 0.010%.siliability superior to those of a prior art material which con up toabout 0.015%. all percentages being by has the same strength due torandom recrystallization weight, and remainder substantially iron. ofthe same percentage. In the steel of this invention, 3. Aluminum coated.cold rolled. low carbon steel the recrystallized grains are at thesurfaces where their having a yield strength of about 90 ksi (620 MN/mductility permits greater elongation of the outer fibers after hot dipaluminum coating. said steel consisting es of the section. sentially offrom about 0.002% to about 0.0l5% car- Once the cold reduced structurehas recrystallized, it is very stable. as shown in FIGS. 8 and 9 whichhave been annealed at l,300F (about 975K) for 4 hours bon. from aboveabout 0.025% to about 0.30% columbium, from about 0.05% to about 0.60%manganese. up to about 0.035% sulfur. oxygen up to about 0.0l0%.

nitrogen up to about 0.012%. aluminum up to about 0.080%. phosphorousand silicon in residual amounts, all percentages being by weight, andremainder sub stantially iron. with at least 0.025% by weight ofuncombined columbium being present as determined by analysis at roomtemperature and calculated from either of formulae 1 and 2 hereinv 4.Hot dip metallic coated, cold rolled. low carbon steel having a yieldstrength of about 20 ksi (138 MN/m") and an Tvalue of at least about1.75 after hot dip metallic coating. said coating being chosen from theclass consisting of aluminum and zinc. said steel either of formulae land 2 herein.

1. HOT DIP METALLIC COATED, HOT ROLLED, LOW CARBON STEEL HAVINGSUBSTANTIALLY NO YIELD POINT ELONGATION AND HAVING TENSILE STRENGTHSRANGING FROM ABOUT 40 TO 70 KIS (275 TO 485 MN/M2) AFTER STRETCHING ANDROLLER LEVELING, THE METALLIC COATING BEING CHOSEN FROM THE CLASSCONSISTING OF ALUMINUM AND ZINC, SAID STEEL CONSISTING ESSENTIALLY OFFROM ABOUT 0.002% TO ABOUT 0.015% CARBON, FROM ABOVE ABOUT 0.025% TOABOUT 0.30% COLUMBIUM, FROM ABOUT 0.05% TO ABOUT 0.06% MANGANES, SULFURUP TO ABOUT 0.035% OXYGEN UP TO ABOUT 0.010% NITROGEN UP TO ABOUT0.012%, ALUMINUM UP TO ABOUT 0.080%, PHOPHORUS AND SILICON IN RESIDUALAMOUNTS, ALL PERCENTAGES BEING BY WEIGHT AND REMAINDER SUBSTANTIALLYIRON, WITH AT LEAST 0.025% BY WEIGHT OF UNCOMBINED COLUMBIUM BEINGPRESENT AS DTERMINED BY ANALYSIS AT ROOM TEMPERATURE AND CALCULATED FROMEITHER OF FORMULAS 1 AND 2 HEREIN.
 2. Hot dip metallic coated steel asclaimed in claim 1, said steel consisting essentially of from about0.005% to about 0.010% carbon, from about 0.08% to about 0.12%columbium, the weight ratio of columbium to carbon being at least about10:1, from about 0.10% to about 0.35% manganese, up to about 0.02%sulfur, oxygen up to about 0.004%, nitrogen up to about 0.006%, aluminumfrom about 0.015% to about 0.020%, substantially all the aluminum beingcombined with oxygen and nitrogen, phosphorus up to about 0.010%,silicon up to about 0.015%, all percentages being by weight, andremainder substantially iron.
 3. Aluminum coated, cold rolled, lowcarbon steel having a yield strength of about 90 ksi (620 MN/m.sup.2)after hot dip aluminum coating, said steel consisting essentially offrom about 0.002% to about 0.015% carbon, from above about 0.025% toabout 0.30% columbium, from about 0.05% to about 0.60% manganese, up toabout 0.035% sulfur, oxygen up to about 0.010%, nitrogen up to about0.012%, aluminum up to about 0.080%, phosphorous and silicon in residualamounts, all percentages being by weight, and remainder substantiallyiron, with at least 0.025% by weight of uncombined columbium beingpresent as determined by analysis at room temperature and calculatedfrom either of formulae 1 and 2 herein.
 4. Hot dip metallic coated, coldrolled, low carbon steel having a yield strength of about 20 ksi (138MN/m.sup.2) and an r value of at least about 1.75 after hot dip metalliccoating, said coating being chosen from the class consisting of aluminumand zinc, said steel consisting essentially of from about 0.002% toabout 0.015% carbon, from above about 0.025% to about 0.30% columbium,from about 0.05% to about 0.60% manganese, up to about 0.035% sulfur,oxygen up to about 0.010%, nitrogen up to about 0.012%, aluminum up toabout 0.080%, phosphorus and silicon in residual amounts, allpercentages being by weight, and remainder substantially iron, with atleast 0.025% by weight of uncombined columbium being present asdetermined by analysis at room temperature and calculated from either offormulae 1 and 2 herein.